I’ve already managed to get my file to validate – Hooray?! I’m confused because many of the errors completely disappeared by making small changes. For example: errors appear to be created by blockquote and em tags (once corrected) vanished from the rest of file. I expected an inconsistency to be, erm, consistent. But no. em and sup tags are throughout my file, but the validator doesn’t care. Certain tags appear to cause issues: I’m looking at you em tag!

This place https://code.google.com/p/epubcheck/wiki/Errors didn’t appear to be much help in figuring out what the errors meant. Ahh well, at least my file has passed validation. If the validator link above passes, then it is extremely likely that the Smashwords meatgrinder will too – as it is based on the same thing.